Vacuum heat treating furnaces utilizing electrical heating elements are well known in the art. A typical vacuum heat treating furnace includes a furnace wall and a hot zone chamber of circular cross-section which houses a series of banks of axially-spaced electrical resistance heating elements suspended from an inner wall of the hot zone chamber by a series of support rods attached to the outer wall of the backing ring through the insulation/radiation shields into the heating elements. A heating element is generally manufactured from graphite, molybdenum or other conducting materials, and generates radiant heat in response to the passage of electrical current therethrough. Typical examples of such furnace support rod designs can be seen in U.S. Pat. Nos. 4,259,538; 4,425,660; 4,559,631; 5,930,285; 6,021,155; 6,023,487; 6,111,908; 6,936,792; 7,514,035; and 8,088,328.
A typical design of the support rods includes electrical insulating components such as ceramic washers or tubing necessary to separate the heating element from the portion of the support rod that is directly affixed to the furnace wall. One drawback with the use of ceramic insulators is the eventual contamination of the ceramic surface from metallic depositions, or metallization, due to outgassing of materials within the heated hot zone or material spilling on or dropping onto the support elements at the bottom of the furnace during processing. The eventual buildup of metal on the ceramic insulators leads to loss of their insulation properties and increased conduction between the heating element and the furnace wall, leading to short circuits and arcing, which can result in furnace operation disruption and possible physical damage to the heating elements and work pieces in the furnace at the time of arcing.
Various support rod designs described in U.S. Pat. Nos. 6,111,908 and 5,930,285 address the use of different arrangements of fixturing within the rod support design to protect the ceramics from metallization. The problem of “short circuiting” between the mounting rods and the heating elements was fully described in U.S. Pat. No. 4,259,538 and further discussed in U.S. Pat. Nos. 5,930,285 and 6,111,908. Shielding by use of molybdenum shields or vaporization shields to prevent material build-up, or metallization, which causes the afore-mentioned short circuits, has also been fully explored in the prior art references, These designs have been used for many years, and not only do they still suffer regularly from metallization issues, but also from failures due to the actual physical design of the support rod itself.
In both prior art U.S. Pat. Nos. 4,259,538 and 6,111,908 the support rod is subjected to drilling for the placement of locking mechanisms to hold the washers and insulation tubing in place. The actual process of drilling three holes into the support rod not only becomes time consuming and costly, but it also provides three locations for stress failure within the rod and also failure of the fasteners inserted into the holes due to repeated heating and cooling during the lifetime of the furnace. These weaknesses lead to loss of stability and movement within the support assembly, and loss of insulating capacity, as the support rods and heating elements are exposed to electrical currents. The result of such fastener damage is short circuits which also lead to operation disruption and possible damaging arcing within the furnace while at high heat temperatures.
As fully described in prior art U.S. Pat. Nos. 4,425,660 and 6,111,908, the disclosures of both of which are incorporated in their entireties herein by reference, under the conditions of a high temperature heat treatment process with relatively long cycling times, a certain amount of molybdenum from moly elements or shields reacts with water vapor inherent in a vacuum furnace even at very high temperatures and leads to the formation of a gaseous MoO3, which deposits on the adjacent ceramic insulators. During subsequent vacuum heat treat cycles, the MoO3 is reduced leaving elemental Mo adhered to the ceramic washers, Subsequent heat treat cycles over time lead to a buildup of molybdenum film on the ceramic washers resulting in increased conductivity on these ceramic washers and loss of the insulation capability of these washers. The resultant conductive nature of these ceramic washers leads to short circuits, as the support rod and the heating element are no longer separated electrically.
The arrangement in U.S. Pat. No. 4,425,660 overcame this issue with the molybdenum shield by providing a pair of graphite shields to be used in lieu of the molybdenum shield. In another embodiment in this patent the graphite liners were secured to the sides of the molybdenum shields that faced the heating element. Other embodiments of the graphite protective liners are described in this patent. However, as described in U.S. Pat. No. 6,111,908, use of the design in U.S. Pat. No. 4,425,660 under long high temperature baking cycles and with alumina type washers, the improvements described in U.S. Pat. No. 4,425,660 shielding packages adequately protect the insulators from such destructive build-up of conductive materials. Other problems with the older: designs in U.S. Pat. No. 4,425,660, and addressed in U.S. Pat. No. 6,111,908, were also covered and described in U.S. Pat. Nos. 6,021,155 and 6,023,487, the disclosures of which are incorporated in their entireties herein by reference.
In the case of U.S. Pat. No. 6,111,908 the use of molybdenum shields was removed completely, and graphite cupped shields were used to protect the insulator means from metal deposition from either the molybdenum elements of from parts in the furnace during heat treating cycles. At the time of the invention described in U.S. Pat. No. 6,111,908 it was believed that at high temperature graphite did not have any reactivity toward the ceramic insulator. It has now been determined that at high temperature and in high vacuum, graphite abutting ceramic insulators can reduce the ceramic oxide, leading to loss of the ceramic material and providing a means to electrically connect the heating element and the rod support. This results in arcing and damage to the furnace.
The present invention addresses the weaknesses of prior art fastener support assemblies which lead to loss of fastener wire, and thus movement of ceramic washers and sleeves, thereby exposing the support rod to electrical contact with the heater element. It also provides a new and improved design which serves to reduce continued contamination of the ceramic washers. Key features of the present invention and weaknesses of the prior art designs are as follows: (1) the support rod is a single solid piece of molybdenum, carbon fiber composite (CFC) or graphite material; (2) the support rod contains no holes or cuts in the material; (3) the cost of manufacturing the support rods is significantly reduced due to elimination of drilling the holes; (4) the cost of using molybdenum wire or rod fasteners that easily become embrittled during repeated heating and cooling cycles is eliminated; (5) drilled holes are a set size, while heating element thickness can vary; the use of a solid rod eliminates the need to modify the standoff arrangement and standardizes the heater element support design to fit a variety of element designs of varied thicknesses; (6) the support rod is threaded on at least one end, or can be threaded on both ends or throughout, and is designed in such a way that a graphite threaded washer can be used to affix the support rod to the outside of the hot zone wall and to the inside heater element; a ceramic washer is used as a separator between the threaded graphite washer and the metal heater element surface; (7) use of graphite nuts in place of molybdenum nuts makes maintenance easier in that the natural lubricity of graphite prevents seizing of the nut and the rod, in which case the nut and rod would be destroyed and would have to be replaced.